What's the difference between the 6mm Grendel and the 6mm ARC?

[Lemonaid]

When did the Grendel get a “60 degree shoulder”? Think you have a math error brother! Same thing for the 6 PPC! Both have 30 degree shoulders, just like the 6mm ARC.

Yep, my mistake. I looked at this diagram and saw 60* https://www.accurateshooter.com/technica...ediagrams/
The two 30 degree shoulders add up to 60. I'll edit my post.